The immune response requires a delicate balance between activation and attenuation. Likewise, therapies for autoimmune diseases and organ transplantation face the challenge of achieving enough immunosuppression to prevent organ rejection or limit autoreactivity without impairing the host's ability to protect against infections and malignancy. Regulatory T-cells (Treg) are an important T-cell subset crucial to self-tolerance, capable dampening or switching off antigen-specific immune responses (1). The transcription factor Forkheadbox-p3 (Foxp3) plays a key role in the development and functions of Treg. Foxp3 is regulated transcriptionally but also by post-translational modifications. We have shown that acetylation of lysine residues within the Foxp3 protein can enhance Treg function (2). Such acetylation also protects Foxp3 from proteasomal degradation and thus contributes to optimal Treg functions. Foxp3 acetylation is regulated by the competing actions of several histone/protein acetyltransferases (HAT) and histone/protein deacetylases (HDAC). We propose to study the role of Sirtuin-1 (Sirt1), a class III HDAC highly conserved across eukaryotic species and an important mediator of cellular metabolism and longevity, in Tregs. We have begun to exploit use of mice with targeted deletions of Sirt1, which is important as mice with global Sirt1 knockout suffer from metabolic problems and shortened lifespan. In addition, we have employed Sirt1 small molecule inhibitors to test the effect of transient Sirt1 inhibition in wild-type mice. Our preliminary data show that the targeted deletion of Sirt1 increases the acetylation and expression of Foxp3, and enhances the immunosuppressive functions of Treg. In addition, deletion of Sirt1 in Tregs, or its pharmacologic inhibition, attenuates allograft rejection and prolongs survival of murine cardiac allografts. Therefore, our central hypothesis is that targeting Sirt1 may have therapeutic value in autoimmunity and transplantation. Our aims are to explore how Sirt1 deletion or inhibition: 1) improves allograft survival and function (in murine recipients with induced diabetes and renal failure); 2) alleviates autoimmunity (in murine inflammatory bowel disease models); and, further, understand 3) distinct molecular mechanisms how Sirt1 influences T-cell biology beyond simply promoting Foxp3 acetylation. Our findings will likely prove important to the development of new immunomodulatory strategies for application in autoimmunity and transplantation. Our proposed studies are also important for an increased understanding of the functions of Sirt1 in immune responses, given the rising attention being given to the Sirt1 activator resveratrol (already available over the counter) and other more potent small molecule Sirt1 activators (3), for therapy of various diseases and for promotion of overall well-being. In addition, Sirt1 inhibitors are under consideration as anti-neoplastic drugs (4). Therefore, we caution that it is critical to learn more about the role of Sirt1 in the immune system as such treatment options are evolving.